Generally, in an electrode of a polymer electrolyte fuel cell, a pore as a supply channel of a reaction gas, a hydrogen ion-conductive polymer electrolyte and a conductive carbon material as an electron conductor form a so-called three-phase interface whose area size exerts an influence on discharge performance of the cell. In order to increase the three-phase interface and to reduce the used amount of noble metal as a catalyst material, attempts have been conventionally made to mix and disperse the hydrogen ion-conductive polymer electrolyte in the catalyst material. For example, a method has been proposed which comprises applying a mixture of a dispersion of a polymer electrolyte and a catalyst material onto a polymer electrolyte membrane and putting the resultant matter and an electrode material together to be hot pressed, and then the catalyst material is reduced (Japanese Examined Patent Publication No. Sho 62-61118 and Japanese Examined Patent Publication No. Sho 62-61119).
Moreover, a method has been proposed in which, after molding of a porous electrode, a dispersion of an ion exchange resin is dispersed over the electrode and then the electrode and the ion exchange resin are hot pressed (Japanese Examined patent Publication No. Hei 2-48632). Further proposed have been: a method in which a resin powder coated with a polymer electrolyte is mixed in an electrode (Japanese Laid-Open Patent Publication No. Hei 3-184266); a method in which a powder of a polymer electrolyte is mixed in an electrode (Japanese Laid-Open Patent Publication No. Hei 3-295172); and a method in which a polymer electrolyte, a catalyst, a carbon powder and a fluorocarbon resin are mixed to form a film so as to serve as an electrode (Japanese Laid-Open Patent Publication Hei 5-36418). U.S. Pat. No. 5,211,984 proposes a method which comprises preparing an ink-like dispersion comprising a polymer electrolyte, a catalyst and a carbon powder with the use of glycerine or tetrabutyl ammonium salt as a solvent, molding the dispersion on a film of polytetrafluoroethylene (hereinafter referred to as PTFE), and then transferring it onto the surface of a polymer electrolyte membrane. This patent further reports a method which comprises substituting the exchange group of the polymer electrolyte membrane with that of a Na type, and applying the aforesaid ink-like dispersion onto the surface of the membrane, followed by heating and drying at 125° C. or higher, to substitute the exchange group again with that of an H type.
A hydrogen ion-conductive polymer is dispersed in the catalyst layer of the polymer electrolyte fuel cell for the purpose of increasing the three-phase interface, as thus described. However, introduction of the hydrogen ion-conductive polymer, namely an increase in insulating material, inhibits electric conductivity. In connection with this, potentials are concentrated on part of an electron conducting channel, causing the hydrogen ion-conductive polymer present in the vicinity of the electron conducting channel to deteriorate. In other words, it is considered that the life characteristic of the polymer electrolyte fuel cell depends on the deterioration in hydrogen ion-conductive polymer due to the potential concentration, and thus the voltage characteristic of the cell deteriorates.
On the other hand, in the electrodes of the fuel cell, an excess fuel gas and oxidant gas which were not consumed in reactions have a role to carry water, permeating from the catalyst layer to a gas diffusion layer, to the outside of the cell. Water present in the fuel cell is important for enhancing conductivity of the polymer electrolyte membrane serving to conduct hydrogen ions and of the hydrogen ion-conductive polymer electrolyte in the catalyst layer. When this water is removed by the excess fuel gas and oxidant gas to the outside of the fuel cell during the operation thereof, the internal resistance of the cell increases and the performance thereof deteriorates.
In order to solve this problem, normally, the fuel gas and the oxidant gas to be introduced to the anode and the cathode are humidified. With the aim of downscaling a humidifying apparatus, however, the humidified amounts of the fuel gas and oxidant gas to be supplied to the fuel cell are small. When the fuel cell is operated for a long time in such a low-humidified state, the amount of the water to be removed by the excess fuel gas and oxidant gas to the outside of the cell increases, causing deterioration in cell performance with the passage of time.
For solving this problem, an attempt has been made to dispose a water-repellent layer between the catalyst layer and the gas diffusion layer of the electrode so as to suppress movement of water to the gas diffusion layer; however, sufficient effects have not yet been obtained.
It is also an object of the present invention to provide a means to suppress the deterioration in cell performance which appears as a problem when the fuel cell is operated in a low-humidified state for a long time.